Piezoelectric device

ABSTRACT

A piezoelectric device that includes: a diaphragm; a supporting part configured to support at least a portion of an end of the diaphragm; a piezoelectric film disposed along a portion supported by the supporting part on the diaphragm, a width of the film along the supported portion being narrower than a width of the portion; a lower electrode disposed at a face of the piezoelectric film on a diaphragm side; and an upper electrode disposed on a face of the piezoelectric film on an opposite side to the diaphragm.

BACKGROUND OF THE INVENTION

The present invention relates to piezoelectric devices.

Electronic apparatuses such as cellular phones may be mounted with aplurality of microphones. For instance, a cellular phone may be providedwith a microphone for detecting ambient sound (environmental sound) fora purpose of noise canceling in addition to a microphone for detecting atransmission voice during a call. As more and more electronicapparatuses are mounted with a plurality of microphones, downsizing ofmicrophones is increasingly demanded.

Against a background like this, in recent years, a microphonemanufactured using a micro electro mechanical systems (MEMS) technology(hereinafter, referred to as a MEMS microphone) has been drawing anattention (for instance, Patent Publication JP 2004-506394 A).

In mounting a microphone onto an electronic apparatus, not onlydownsizing of a microphone but also sensitivity enhancement of themicrophone is required. Sensitivity enhancement is required also for aMEMS microphone.

SUMMARY OF THE INVENTION

The present invention has been made in consideration of suchcircumstances, and an object thereof is to enhance the sensitivity of aMEMS microphone.

A piezoelectric device related to one aspect of the present inventionincludes: a diaphragm; a supporting part configured to support at leasta part of an end of the diaphragm; a piezoelectric film disposed on thediaphragm along a portion supported by the supporting part, a width ofthe film along the supported portion being narrower than a width of theportion; a lower electrode disposed on a surface of a diaphragm side ofthe piezoelectric film; and an upper electrode disposed on a face of thepiezoelectric film on an opposite side to the diaphragm.

According to the present invention, the sensitivity of a MEMS microphonecan be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing an appearance of a piezoelectric device of oneembodiment of the present invention;

FIG. 2 is a cross-sectional view showing the piezoelectric device;

FIG. 3 is a view showing one example of a wiring of an electrode of thepiezoelectric device;

FIG. 4 is a graph showing one example of a relation between a width of apiezoelectric part and voltage sensitivity;

FIG. 5 is a graph showing one example of a relation between a width ofthe piezoelectric part and generated energy;

FIG. 6 is a graph showing one example of a relation between temperatureand Young's modulus;

FIG. 7 is a view showing another configuration example of thepiezoelectric device;

FIG. 8 is a view showing another configuration example of thepiezoelectric device;

FIG. 9 is a view showing one configuration example of a diaphragm inwhich a vicinity of a center thereof is made thin;

FIG. 10 is a view showing another configuration example of thepiezoelectric device;

FIG. 11 is a view showing another configuration example of thepiezoelectric device; and

FIG. 12 is a view showing another configuration example of thepiezoelectric device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of the present invention is described below withreference to the drawings. FIG. 1 is a view showing an appearance of apiezoelectric device of one embodiment of the present invention. Apiezoelectric device 100 is a device for configuring a MEMS microphonethat converts sound pressure into an electrical signal, and includes adiaphragm 110, a supporting part 111, and piezoelectric parts 112. Thepiezoelectric device 100 is divided into two by a minute slit 113 ofabout 1 μm or less wide for instance.

The diaphragm 110 is a thin film that vibrates due to sound pressure andcan be formed of silicon (Si). The diaphragm 110 has a substantiallyrectangular shape, wherein lower parts of a facing set of sides 114 and115 are supported by the supporting part 111. In other words, thediaphragm 110 has a both-end supported beam structure. It is beneficialthat the diaphragm 110 is degenerate silicon, and has a function as alower electrode of the piezoelectric part 112 as described later. Whatis called a degenerate silicon or degenerate semiconductor doped with adopant in high concentration (1×10¹⁹ cm⁻³ or over). To be more precise,by doping phosphorus (P), arsenic (As), or antimony (Sb) at aconcentration of 1×10¹⁹ cm⁻³ or over into Si as an n-type dopant (adonor), a degenerate semiconductor can be formed. A degeneratesemiconductor may also be formed by doping a p-type dopant (an acceptor)into Si.

The piezoelectric parts 112 are disposed along the portion supported bythe supporting part 111 on the diaphragm 110. As shown in FIG. 1, awidth (A) of the piezoelectric part 112 (a width of a piezoelectric film210 as described later) is narrower than a width (B) of the portionsupported by the supporting part 111 on the diaphragm 110 (in otherwords, a width of the side 114). For instance, the width (A) of thepiezoelectric part 112 may be about 100 μm, and the width (B) of theportion supported by the supporting part 111 on the diaphragm 110 may beabout 300 μm. Although four piezoelectric parts 112 are disposed on thediaphragm 110 in the configuration shown in FIG. 1, the number ofpiezoelectric parts 112 is not limited to this. In the configurationshown in FIG. 1, although ends of the piezoelectric parts 112 aredisposed on the sides 114 and 115, ends may be disposed separately fromthe sides 114 and 115.

FIG. 2 is a cross-sectional view of the piezoelectric device 100 at aline X-Y shown in FIG. 1. The supporting part 111 includes a substrate200 and an insulating layer 201.

The substrate 200 is formed of silicon (Si) for instance. The insulatinglayer 201 is formed of silicon oxide (SiO₂) for instance. The diaphragm110 is formed on the supporting part 111 formed in this manner.

Each of the piezoelectric parts 112 disposed along the portion supportedby the supporting part 111 on the diaphragm 110 includes a piezoelectricfilm 210, an upper electrode 211, and wirings 212 and 213.

The piezoelectric film 210 is disposed on the diaphragm 110 so as to bevibrated in association with vibration of the diaphragm 110. Thepiezoelectric film 210 is a thin film of a piezoelectric body thatconverts force applied by the vibration to voltage, and is formed ofscandium doped aluminum nitride (ScAlN) for instance. ScAlN is formed bysubstituting a part of aluminum (Al) in aluminum nitride (AlN) withscandium (Sc). For instance, ScAlN used for the piezoelectric film 210may be formed by substituting Al with Sc so that Sc becomes about 40atom % when atomic concentration that is a sum of the number of Al atomsand the number of Sc atoms is assumed to be 100 atom %. The thickness ofthe piezoelectric film 210 may be about 500 nm for instance. A ratio ofa width (D) of a vibration portion of the piezoelectric film 210 to awidth (C) from a center of the diaphragm 110 to the supporting part 111may be about 40% for instance. The width (C) may be about 300 μm and thewidth (D) may be about 120 μm for instance.

The upper electrode 211 is disposed on an upper side of thepiezoelectric film 210. The upper electrode 211 is a metal electrode andmay be formed of aluminum (Al) for instance, and may have a thickness ofabout 50 nm. The upper electrode 211 may have tensile stress. Since thepiezoelectric film 210 formed of ScAlN has a compressive stress, byallowing the upper electrode 211 to have a tensile stress, a stress atthe piezoelectric part 112 is corrected and a deformation of thediaphragm 110 can be suppressed.

The wiring 212 is electrically coupled to the upper electrode 211. Thewiring 213 is electrically coupled to the lower electrode (the diaphragm110). The wirings 212 and 213 are formed by using gold (Au), platinum(Pt), titanium (Ti), aluminum (Al), or the like for instance.

In the piezoelectric device 100 of the configuration described above,the piezoelectric film 210 vibrates in association with a vibration ofthe diaphragm 110 caused by sound pressure. Voltage corresponding to thevibration of the piezoelectric film 210 is output through the wirings212 and 213 of the piezoelectric body 112. As shown in FIG. 1, fourpiezoelectric bodies 112 are provided in the piezoelectric device 100.The four piezoelectric bodies 112 can be electrically coupled inparallel as shown in FIG. 3 for instance. The coupling shown in FIG. 3is just an example and a form of coupling of the piezoelectric bodies112 is not limited to this.

In the piezoelectric device 100, as shown in FIG. 1, the width (A) ofthe piezoelectric part 112 (the width of the piezoelectric film 210) isnarrower than the width (B) of the portion supported by the supportingpart 111 of the diaphragm 110 (in other words, the width of the side114). Such a structure increases the stress applied to the piezoelectricpart 112 by the vibration of the diaphragm 110 caused by sound pressurecompared to the case where the piezoelectric part 112 has the same widthas the diaphragm 110 (in other words, the width (A) of the piezoelectricpart 112=the width (B) of the diaphragm 110). Consequently, the stressapplied to a unit area of the piezoelectric part 112 becomes large, andthe voltage sensitivity and the generated energy of the piezoelectricpart 112 can be increased. In other words, sensitivity of the MEMSmicrophone configured by using the piezoelectric device 100 can beimproved.

FIG. 4 is a graph showing one example of a relation between the width ofthe piezoelectric part 112 and the voltage sensitivity at thepiezoelectric device 100. In FIG. 4, the horizontal axis represents aratio (%) of the width (A) of the piezoelectric part 112 to the width(B) of the diaphragm 110, and the vertical axis represents the voltagesensitivity (mV/Pa) showing output voltage (mV) per sound pressure (Pa)at the piezoelectric part 112. As shown in FIG. 4, the voltagesensitivity increases as the ratio of the width of the piezoelectricpart 112 to that of the diaphragm 110 decreases. Accordingly, at thepiezoelectric device 100 of the embodiment, the voltage sensitivity canbe improved.

FIG. 5 is a graph showing one example of a relation between the width ofthe piezoelectric part 112 and the generated energy. In FIG. 5, thehorizontal axis represents the ratio (%) of the width (A) of thepiezoelectric part 112 to the width (B) of the diaphragm 110, and thevertical axis represents the generated energy (fJ/Pa) per sound pressureat the piezoelectric part 112. As shown in FIG. 5, the generated energyincreases as the ratio of the width of the piezoelectric part 112 tothat of the diaphragm 110 decreases. Accordingly, at the piezoelectricdevice 100 of the embodiment, the generated energy can be enlarged.

When the width of the piezoelectric part 112 becomes narrow, acapacitance value of the piezoelectric part 112 becomes small. When thecapacitance value becomes small, impedance mismatching with an amplifiercircuit may be likely to occur due to an impedance increase, or aninfluence of parasitic capacitance may be likely to become large.Therefore, the width (A) of the piezoelectric part 112 is determined bytaking account of a trade-off between improvement of the voltagesensitivity and an increase in the impedance and such.

When Young's modulus of the diaphragm 110 changes with temperature, thevoltage sensitivity of the piezoelectric device 100 also changes. Inthis respect, in the embodiment, since the diaphragm 110 is formed of adegenerate semiconductor, a change of Young's modulus of the diaphragm110 with temperature can be suppressed. In other words, a change of thevoltage sensitivity of the piezoelectric device 100 can be suppressed.

FIG. 6 is a graph showing one example of a relation between temperatureand Young's modulus. In FIG. 6, the horizontal axis representstemperature (° C.) and the vertical axis represents Young's modulus(GPa). In FIG. 6, four temperature characteristics (P1) to (P4) of thediaphragm 110 with different doping concentrations are shown. P1 in FIG.6 shows a temperature characteristic when nothing is doped into Si. P2in FIG. 6 shows a temperature characteristic when an n-type dopant isdoped into Si at a concentration of 1×10¹⁹ cm⁻³. P3 in FIG. 6 shows atemperature characteristic when the n-type dopant is doped into Si at aconcentration of 5×10¹⁹ cm⁻³. P4 in FIG. 6 shows a temperaturecharacteristic when the n-type dopant is doped into Si at aconcentration of 8×10¹⁹ cm⁻³. As shown in FIG. 6, in comparison withYoung's modulus in a case P1 where nothing is doped into Si, by makingthe doping concentration in Si 1×10¹⁹ cm⁻³ or more, in other words, bymaking Si into a degenerate semiconductor, the change in Young's modulusdue to temperature change can be suppressed. With the diaphragm 110being formed of a degenerate semiconductor, the change of the voltagesensitivity of the piezoelectric device 100 can be suppressed.

FIG. 7 is a view showing another configuration example of thepiezoelectric device. As for the same components as those of thepiezoelectric device 100 shown in FIG. 1, the same reference numeralsare given and descriptions are omitted. As shown in FIG. 7, in thepiezoelectric device 700, lower parts of all sides 710 to 713 of thediaphragm 110 are supported by the supporting part 111. In other words,the diaphragm 110 has an entire circumference supported beam structure.A configuration of a cross-section at an X-Y line shown in FIG. 7 isequivalent to the configuration shown in FIG. 2.

The piezoelectric device 100 shown in FIG. 1 has the both-end supportedbeam structure, wherein the diaphragm 110 is provided with the slit 113.For that reason, in the piezoelectric device 100, while the diaphragm110 is easily bendable and the voltage sensitivity can be improved, thediaphragm 110 may be deformed due to stresses of the piezoelectric films210 and the upper electrodes 211, which may change an amount of soundleakage from the slit 113 and may cause variations in the voltagesensitivity. On the contrary, as shown in FIG. 7, the piezoelectricdevice 700 is not provided with the slit 113 of the piezoelectric device100. Therefore, in the piezoelectric device 700, no sound leakage fromthe slit occurs and the variations in the voltage sensitivity can besuppressed.

FIG. 8 is a view showing another configuration example of thepiezoelectric device. As for the same components as those of thepiezoelectric device 100 shown in FIG. 1, the same reference numeralsare given and descriptions are omitted. As shown in FIG. 8, in apiezoelectric device 800, a slit 810 is provided along a substantialcenter line 810 substantially parallel to the sides 114 and 115supported by the supporting part 111 in addition to the slit 113. Thediaphragm 110 is divided into four by the slits 113 and 810. In otherwords, the diaphragm 110 has a cantilever structure. By making thediaphragm 110 the cantilever structure, the diaphragm 110 becomes moreflexible than in the case of the piezoelectric device 100 shown in FIG.1, and the voltage sensitivity can be improved.

FIG. 9 is a view showing one configuration example in which a vicinityof a center of the diaphragm 110 is made thin in the piezoelectricdevice 100 shown in FIG. 1. As shown in FIG. 9, a thickness of a region901 in the vicinity of the center of the diaphragm 110 can be formedthinner than a thickness of a region 900 of the diaphragm 110, where thepiezoelectric film 210 is disposed. In this manner, by thinning thethickness of the region 901 in the vicinity of the center of thediaphragm 110, the diaphragm 110 is made to be easily bendable, and thevoltage sensitivity can be improved. Since the piezoelectric device 800shown in the FIG. 8 is not provided with the slit 810, the deformationof the diaphragm 110 is suppressed, and the variations in the voltagesensitivity are suppressed.

Changing the thickness of the region 900 in the diaphragm 110, where thepiezoelectric film 210 is disposed, changes a state of expansion andcontraction of the piezoelectric film 210 resulting from the vibrationof the diaphragm 110 and changes voltage output characteristics of thepiezoelectric part 112. More specifically, thinning the thickness of theregion 900 in the diaphragm 110, where the piezoelectric film 210 isdisposed, when the piezoelectric film 210 bends downwardly for instance,may increase an amount of contraction of an under side of thepiezoelectric film 210, may cancel out voltage output resulting fromexpansion of an upper side of the piezoelectric film 210, and may reducethe output voltage from the piezoelectric part 112. For this reason,thinning only the region 901 in a vicinity of a center without changingthe thickness of the region 900 can improve the voltage sensitivitywithout influencing the voltage output characteristics of thepiezoelectric part 112.

Not only in the configuration shown in FIG. 1 but also in theconfigurations shown in FIG. 7 and FIG. 8, a region in the vicinity ofthe center of the diaphragm 110 may be thinned.

FIG. 10 is a view showing another configuration example of thepiezoelectric device. As for the same components as those of thepiezoelectric device 100 shown in FIG. 1, the same reference numeralsare given and descriptions are omitted. As shown in FIG. 10, thediaphragm 110 has a substantially circular shape in a piezoelectricdevice 1000. In this case also, the width (A) of the piezoelectric part112 is made narrower than the width (B) of the portion supported bysupporting part 111 in the diaphragm 110 of the piezoelectric part 112.In this manner, the diaphragm 110 may have not only a substantiallyrectangular shape but also an arbitrary shape. Although fourpiezoelectric parts 112 are disposed on the diaphragm 110 in FIG. 10,the number of the piezoelectric parts 112 is not limited to this but maybe any number. For instance, as shown in FIG. 11 three piezoelectricparts 112 may be disposed on the diaphragm 110 having a substantiallycircular shape.

FIG. 12 is a view showing another configuration example of thepiezoelectric device. As for the same components as those of thepiezoelectric device 100 shown in FIG. 2, the same reference numeralsare given and descriptions are omitted. As shown in FIG. 2, thediaphragm 110 is used as a lower electrode of the piezoelectric part 112in the piezoelectric device 100. On the other hand, in a piezoelectricdevice shown in FIG. 12, a lower electrode 1210 is provided separatelyfrom the diaphragm 110. The lower electrode 1210 is a metal electrodeand may be formed of aluminum (Al) for instance, and may have athickness of about 50 nm.

The embodiments of the present invention have been described above.According to the embodiments, the piezoelectric device is formed so thatthe width (A) of the piezoelectric part 112 is narrower than the width(B) of the portion supported by the supporting part 111 in the diaphragm110. This increases a stress applied to a unit area of the piezoelectricpart 112, and enables enhancement of the voltage sensitivity and thegenerated energy in the piezoelectric part 112. In other words, thesensitivity of a MEMS microphone configured by using the piezoelectricdevice 100 can be improved.

According to the embodiment, the diaphragm 110 can be formed of adegenerate semiconductor. Thereby, variations in the Young's modulus ofthe diaphragm 110 with temperature can be suppressed, and variations inthe voltage sensitivity of the piezoelectric device with temperature canbe suppressed.

According to the embodiment, the diaphragm 110 formed of a degeneratesemiconductor can be used as a lower electrode of the piezoelectric part112. Thereby, the piezoelectric device 100 can be downsized as comparedwith a case where the lower electrode is formed separately from thediaphragm 110.

According to the embodiment, as shown in FIG. 1, making the diaphragm110 have the both-end supported beam structure makes it possible to makethe diaphragm 110 easily bendable and to improve the voltage sensitivityof the piezoelectric device.

According to the embodiment, as shown in FIG. 8, when making thediaphragm 110 have the both-end supported beam structure, thesubstantial center line 810 substantially parallel to the sides 114 and115 of the supporting part 111 makes the diaphragm 110 have a separateconfiguration, thereby, making the diaphragm 110 more bendable and beingable to improve the voltage sensitivity of the piezoelectric device.

According to the embodiment, as shown in FIG. 7, making the diaphragm110 have the entire circumference supported beam structure makes itpossible to suppress the deformation of the diaphragm 110 and tosuppress the variations in the voltage sensitivity caused by thedeformation of the diaphragm 110.

According to the embodiment, as shown in FIG. 9, making the region 901in the vicinity of the center of the diaphragm 110 thin makes itpossible to make the diaphragm 110 more bendable and to improve thevoltage sensitivity of the piezoelectric device.

According to the embodiment, it is possible to make the upper electrode211 formed at an upper side of the piezoelectric film 210 having acompressive stress have a tensile stress. Thereby, the stress at thepiezoelectric part 112 is corrected, and the deformation of thediaphragm 110 is suppressed.

The embodiment is for facilitating comprehension of the presentinvention, and not for comprehending by limiting the present invention.The present invention can be changed and/or improved without beingdeviated from the gist thereof. The present invention includes alsoequivalents thereof.

For instance, in the embodiment, although an example is described inwhich the piezoelectric device is used as a MEMS microphone by vibratingthe diaphragm by sound pressure, uses of the piezoelectric device is notlimited to this, and is usable for a sensor that detects vibration of amedium in the surrounding of the piezoelectric device.

I (we) claim:
 1. A piezoelectric device comprising: a diaphragm having afirst surface and a second surface; a supporting part adjacent thesecond surface of the diaphragm and configured to support at least aportion of an end of the diaphragm; a piezoelectric film adjacent thefirst surface of the diaphragm and disposed along the portion of the endof the diaphragm supported by the supporting part, a width of thepiezoelectric film along the supported portion being narrower than awidth of the supported portion; and an electrode disposed on a surfaceof the piezoelectric film opposite to the diaphragm.
 2. Thepiezoelectric device according to claim 1, wherein the diaphragm isformed of a degenerate semiconductor.
 3. The piezoelectric deviceaccording to claim 2, wherein the electrode is an upper electrode andthe diaphragm is configured to function as a lower electrode for thepiezoelectric film.
 4. The piezoelectric device according to claim 1,wherein the electrode is an upper electrode and the piezoelectric deviceincludes a lower electrode disposed on a surface of the diaphragm. 5.The piezoelectric device according to claim 1, wherein the diaphragm hasa substantially rectangular shape.
 6. The piezoelectric device accordingto claim 5, wherein the supporting part is configured to support a pairof opposing sides of the diaphragm.
 7. The piezoelectric deviceaccording to claim 6, further comprising a plurality of piezoelectricfilms disposed along each side of the pair of sides of the diaphragm. 8.The piezoelectric device according to claim 7, wherein the diaphragm isdivided along a first substantial center line substantially parallel tothe pair of opposing sides.
 9. The piezoelectric device according toclaim 8, wherein the diaphragm is further divided along a secondsubstantial center line substantially perpendicular to the pair ofopposing sides.
 10. The piezoelectric device according to claim 5,wherein the supporting part is configured to support all sides of thediaphragm, and a plurality of piezoelectric films are disposed alongeach side of all the sides of the diaphragm.
 11. The piezoelectricdevice according to claim 1, wherein the diaphragm is thinner at acenter region thereof than a region thereof where the piezoelectric filmis disposed.
 12. The piezoelectric device according to claim 1, whereinthe piezoelectric film has a compressive stress, and the upper electrodehas a tensile stress.
 13. The piezoelectric device according to claim 1,wherein the supporting part includes a supporting part and an insulatinglayer.
 14. The piezoelectric device according to claim 13, wherein theinsulating layer is adjacent the diaphragm.
 15. The piezoelectric deviceaccording to claim 14, wherein the insulating layer is silicon oxide.16. The piezoelectric device according to claim 1, further comprising aplurality of piezoelectric films adjacent the first surface of thediaphragm, the plurality of piezoelectric films being electricallycoupled in parallel.
 17. The piezoelectric device according to claim 1,wherein the diaphragm has a substantially circular shape.